Hellmuth has made a career out of calling peoples' bluffs. So far, he's won 14 bracelets at the World Series of Poker and a total of $21,971,705 at live tournaments.

Hellmuth partly credits his ability to read people as the secret to his success.

"Success in the game is 70% reading people and only 30% reading the cards," he wrote in the book "Read 'Em and Reap," which he co-authored with writer Marvin Karlins and former FBI agent Joe Navarro.

The "Poker Brat" author has picked up one trick to help him get a better read on people's true feelings — and detect potential lies. It's a simple test anyone can use, whether you're dealing with someone bluffing over a hand of cards or a chronic workplace liar.

To paraphrase the old cliché, the eyes are windows into how we're feeling. In "Read 'Em and Reap," Navarro writes about the subtle clues our pupils give off: "When we like something, our pupils dilate; when we don't, they constrict."

Pupils can change size due to all sorts of stimuli, including drug use, attraction, fear, and changes in light levels, The Scientist reported. Hellmuth said it's important to assess the context of the situation and trust your gut if you're going to use this trick.

"I tend to teach people to just kind of trust their instincts," he said.

People have little ability to control how their pupils react to stimuli, which is what makes watching someone's pupils a great tell. Of course, changes in pupil diameter signal a person's emotional state — not dishonesty. But if an individual's words don't match their eyes, you might want to take note.

For example, if your friend gets a call that causes their pupils to contract but they claim they're fine afterward, something might be off.

Hellmuth himself wrote about a situation in "Read 'Em and Reap" where he was able to fold and miss out on a major loss after realizing his opponent had a full house. How could he tell? His competitor's eyes dilated when he received a certain card during the tournament.

Of course, life's not a poker tournament, and it's not always good manners to stare at someone for an extended period of time. But next time you're having a tough conversation with someone, watch their eyes.

"You can always stare them in the eye and ask them a question," Hellmuth said.